Reactions of Alcohols

[Pages:24]Reactions of Alcohols Alcohols are versatile organic compounds since they undergo a wide variety of transformations ? the majority of which are either oxidation or reduction type reactions.

Normally: Oxidation is a loss of electrons; Reduction is a gain of electrons.

But in organic terms:

Oxidation: loss of H2; addition of O or O2; addition of X2 (halogens).

Reduction: addition of H2 or H-; loss of O or O2; loss of X2.

Neither an oxidation nor reduction: Addition or loss of H+, H2O, HX.

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Oxidation of Alcohols Primary and secondary alcohols are easily oxidized by a variety of reagents.

Secondary Alcohols The most common reagent used for oxidation of secondary alcohols to ketones is chromic acid, H2CrO4.

Chromic acid is produced in situ by reaction of sodium dichromate, sulfuric acid and water. Na2Cr2O7 + H2O + 2H2SO4 2 H2CrO4 + 2 NaHSO4

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Mechanism of oxidation

The alcohol and chromic acid produce a chromate ester, which then reductively eliminates the Cr species. The Cr is reduced (VI IV), the alcohol is oxidized.

Oxidation of Primary Alcohols Primary alcohols are easily oxidized just like secondary alcohols, and the INITIAL product of oxidation is an aldehyde.

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However, the aldehyde can also be easily oxidized to an acid, and this `over-oxidation' is a practical problem. E.g.

A common reagent that selectively oxidizes a primary alcohol to an aldehyde (and no further) is pyridinium chlorochromate, PCC.

N: CrO3, HCl (PCC)

E.g.

Tertiary Alcohols These are resistant to oxidation because they have no hydrogen atoms attached to the oxygen bearing carbon (carbinol carbon).

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Other Oxidizing Reagents Potassium permanganate is a cheaper but stronger oxidizing agent, and conditions must be controlled carefully.

KMnO4, base OH

OH O

Thermal dehydrogenation is the cheapest method of oxidation but the high temperatures involved limit the applicability of this method.

H

OH CuO, 300oC

O

Reduction of Alcohols Normally an alcohol cannot be directly reduced to an alkane in one step.

The ?OH group is a poor leaving group so hydride displacement is not a good option ? however the hydroxyl group is easily converted into other groups that are superior leaving groups, and allow reactions to proceed.

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One such conversion involves tosyl chloride, and the formation of a tosylate. These compounds undergo substitution and elimination very easily, often more reactive than alkyl halides.

Cyclohexanol will not reduce with LiAlH4, but the corresponding tosylate reduces to cyclohexane very easily.

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Tosylate Esters

Tosylate esters (tosylates) are typically formed from alcohols with reaction with Ts-Cl and pyridine (py). Tosylate groups undergo a variety of SN2 reactions.

The tosylate is such a good leaving group because it is a stable anion.

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The tosylate is such a good leaving group because it is a stable anion. Common SN2 transformations of Tosylates:

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